Optical communication with pre-compensation for odd order distortion in modulation and transmission

ABSTRACT

An in-line pre-compensation circuit distorts an electronic information signal prior to using the signal to modulate a laser beam in order to compensate for distortions resulting from transmitting the resulting optical signal through an optical fiber. The pre-compensation circuit is dynamically adjustable for different lengths of optical fiber for simple installation and maintenance of the system. The adjustment can be made from a front panel of a laser transmitter during operation. Even though the optical signal is more distorted at the output of the laser transmitter, the optical signal that arrives at the receiver is less distorted.

FIELD OF THE INVENTION

[0001] The invention is related to the fields of broadband cabletelevision systems and is most closely related to laser opticalcommunication links for such systems.

BACKGROUND OF THE INVENTION

[0002] In a cable television system, television programs are provided ata central head-end. The programs are distributed from the head-endthrough optical fiber tree networks to a multitude of local nodes inrespective communities, and then further distributed from the localnodes through coaxial cable tree networks to customer interface units(CIUs) also called cable terminations. Currently, many of these systemsare beginning to provide other communication services such as telephoneservice and/or computer networking services (e.g. internet connection)through the cable television system. Telephone and computer networkingservices require bi-directional communication in the cable televisionsystem. Forward data signals are transmitted similarly to televisionsignals, as described above, and return data signals are transmittedthrough the same path in the reverse direction. That is, return signalsare collected from the CIUs through the coaxial cable tree networks tothe local nodes, back through the local nodes, and back through theoptical fiber tree network to the head-end.

[0003] At the head-end, a multitude of electronic forward informationsignals for the television, telephone, and computer communications areused to modulate respective carrier signals of different frequencies.The modulated carrier signals are combined into an electronic forwardsignal that is used to modulate a forward laser beam to produce anoptical forward signal carried by the forward laser beam. The modulatedlaser beam, carrying the optical forward signal, is transmitted throughan optical fiber tree network to a multitude of the local nodes. At eachlocal node an optical detector coverts the optical forward signal backinto an electronic forward signal. The reconverted electronic forwardsignal is transmitted from the local nodes through a coaxial conductortree network to CIUs at homes and businesses of customers.

[0004] Telephone and computer equipment of the customer, are connectedto the CIUs by the customers and the customer's equipment produce returnsignals that are transmitted by the CIUs into the coaxial tree. Thereturn signals are multi-carrier modulated signals similar to theforward signals. The return signals travel back through the coaxial treenetwork to the local nodes. In the local nodes, the return signals areseparated from the forward signals by diplex filters. The separatedreturn signals are used to modulate a return laser beam to produce anoptical return signal carried by the return laser beam. The opticalreturn signal is transmitted back through the optical fiber tree networkto the head-end where the optical return signals are converted back intoelectronic return signals by an optical detector for the return signals.The electronic return signals are demodulated and used for telephone andcomputer communications.

[0005] Those skilled in the art are directed to the following citations.U.S. Pat. No. 4,992,745 to Blauvelt discloses a pre-distortion networkfor compensating for second, third, and higher order distortion in atransmission device such as a semiconductor laser. U.S. Pat. No.5,257,124 to Glaab discloses dual optical links to cancel out even orderdistortion. U.S. Pat. No. 5,430,568 to Little discloses a system inwhich 4 independent lasers each transmit different respectivemulti-carrier signals having different respective frequency bands ofless than one octave each. At optical receivers, second orderdistortions are filtered out of each of the 4 signals and then thesignals are combined into a single 54-500 MHz multi-carrier signal. Twopairs of lasers are used to transmit the 4 signals. For each pair oflasers, a first laser with a wavelength of 1310 nm transmits a firstsignal through a first fiber and a second laser with a wavelength of1550 nm transmits a second signals through a second fiber; andwavelength division multiplexing (WDM) is used to combine the twosignals from the first and second fiber into a first common 1310 zerodispersion fiber. Prior to reception, WDM is used to separate the firstand second signals back into separate third and fourth fibers andseparate respective receivers are provided to receive each signal.Electrical and/or optical compensating elements are provided tocompensate for distortion due to dispersion which is not eliminated bygrouping of frequencies discussed above. An electronic compensatingelement in the input of each laser need only compensate for third orderdistortion since the second order distortions are filtered out. Opticalcompensating elements in the second or third fiber compensate for thedispersion of the 1550 nm signal in the 1310 nm zero distortion fiber.The optical compensating elements may be dispersion compensating opticalfibers having dispersion profiles opposite to the dispersion profileexperienced by the optical signals when transmitted over standard 1310nm optical fibers to the receiver location. Such profiles representsecond and third order harmonic distortion, known in the art ascomposite second order and composite triple beat, respectively.

[0006] The above references are hereby incorporated herein in whole byreference.

SUMMARY OF THE INVENTION

[0007] In one embodiment of the invention herein, an electronicinformation signal is used to modulate a laser beam resulting in anoptical signal that is transmitted through an optic fiber tree networkto an optical detector that converts the optical signal back into anelectronic information signal. A pre-compensation circuit distorts theelectronic information signal prior to using the signal for modulatingthe laser beam. The pre-compensation circuit compensates for odd orderdistortion due to transmitting the optical signal through the opticalfiber as well as the odd order distortion due to using the electronicsignal for modulating the laser beam. The optical signal produced by themodulation of the laser beam is more distorted at the laser beammodulator than at an optical detector at a remote end of the opticalfiber. As the distorted signal travels through the optical fiber itbecomes less distorted. The pre-compensation circuit and the laser arecomponents of an optical transmitter for transmitting an inputinformation signal. The optical detector is a component of an opticalreceiver for outputting an electronic information signal thatapproximately duplicates the input information signal at thetransmitter. The third order distortion of the pre-compensation circuitis selected so as to reduce the total odd order distortion in anelectronic signal that is output from the optical receiver.

[0008] Preferably, the laser is directly modulated distributed feedback(DFB) laser which transmits at an optical wavelength selected between1500 to 1610 nm and the optic fiber has approximately zero dispersion atapproximately 1310 nm. The pre-compensation circuit also compensates foreven order distortions due to the modulation of the laser beam and dueto the transmission through the optic fiber.

[0009] Preferably, the pre-compensation circuit also compensates fordistortions due to the optical detector converting the optical signalsback into electronic signals. The pre-compensation circuit alsocompensates for electronic amplification required prior to lasermodulation, electronic amplification required for the output electronicsignal after optical detection, and optical amplification required forthe optical signal.

[0010] In another embodiment of the invention, a pre-compensationcircuit compensates for odd order distortion due to transmitting anoptical signal through an optical fiber, and the level of distortionprovided by the pre-compensation circuit is adjustable depending on thelength of the optical fiber.

[0011] Preferably, the pre-compensation circuit is automaticallyadjustable depending on a return signal from the receiver which iscompared to a portion of the input electronic signal. Also, thepre-compensation circuit includes an input at a front panel for manuallychanging the level of distortion and an output from which the length ofthe fiber, for which the pre-compensation circuit is adjusted, can bedetermined.

[0012] Those skilled in the art can understand the invention andadditional objects and advantages of the invention by studying thedescription of preferred embodiments below with reference to thefollowing drawings that illustrate the features of the appended claims:

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 illustrates a network of the invention with an opticaltransmitter and optical receiver interconnected by an optical fiber.

[0014]FIG. 2 shows another embodiment of the invention with an opticaltransmitter and optical receiver for the same node.

[0015]FIG. 3 illustrates a head-end of a cable television network of theinvention.

[0016]FIG. 4 illustrates a hub and local nodes of the cable televisionnetwork of FIG. 3.

[0017]FIG. 5 shows customer interface units of the cable televisionnetwork of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In the example embodiment of an optical communication link 100shown in FIG. 1, an optical transmitter 101 transmits an optical signalto optical receiver 102 through optical fiber 103. The opticaltransmitter receives an electronic output information signal throughtransmission line 110 which is directed to pre-compensator 111 forcompensating for odd order distortion. The pre-compensator includes afirst in-line compensator 112 to distort the output electronic signalfor compensating for odd order distortion during modulation of a laserbeam using the electronic output signal, and a second in-linecompensator 113 to further distort the output electronic signal forcompensating for odd order distortion during the transmission of theoptical signal through the optical fiber. The pre-compensator alsoincludes an adjuster 114 with an output to the second compensator foradjusting the distortion depending on the length of optical fiber 103.The adjuster has a input 115 from front panel 140 for manually inputtinga signal depending on the length of the optical fiber. An input 116 isalso provided for an automatic adjustment which may be based on a returnsignal from the receiver 102 as described below with reference to FIG. 2and an output 117 for providing a signal from which the optical fiberlength, for which the second compensator is adjusted, can be determined.

[0019] Circuits for compensating for odd order distortions are wellknown in the art and those skilled in the art can provide the first andsecond compensators based on these known circuits. The circuits for thefirst and second compensator and for the adjuster can be separatecircuits as shown, or a single circuit performing the function of two ormore of these separate circuits can be provided. The length input signalmay be a manual input such as an adjustment of a potentiometer at thefront panel as shown or a return signal from the optical receiver asdescribed below.

[0020] A distorted signal 120 is output from pre-compensator 111 anddirected to laser 121 and used to directly modulate the laser so as tomodulate laser beam 122 in order to convert the distorted electronicsignal into an optical signal carried by the laser beam. An opticssystem 123 contains lenses to focus the laser beam into the end of anoptical fiber for transmitting the optical signal through the fiber toreceiver 102. For an amplitude modulated virtual side band (AM-VSB)signal or a quadrature phase key signal with 32 levels (QSK-32) signal,the optical signal may be transmitted through the optical fiber from theoptical transmitter to the optical receiver through a distance of over100 km.

[0021] Preferably, the laser is a directly modulated laser as shown, butan external modulator could be provided. Also, the laser is preferably adistributed feedback (DFB) laser as shown, but other types of laserscould be used optical systems to direct laser beams into optical fibersare well known. The type of optical fiber is preferably single mode withapproximately zero dispersion at an optical wavelength of about 1310 nm.Part of the invention lies in the recognition that transmission ofoptical signals through zero dispersion fiber results in odd orderharmonics which become significant at the transmission distances whenthe steps disclosed herein to reduce even order and other odd orderdistortions are implemented. Part of the invention is that the opticalsignal in the fiber at the transmitter contains more odd orderdistortions than the optical signal in the fiber at the receiver.

[0022] Optical receiver 102 includes an optical detector 130 connectedat another end of optical fiber 103, which is remote from the opticaltransmitter. The optical detector converts the optical signal into anelectronic information signal. The optical receiver also includes anin-line post-compensator 131 to linearize the electronic signal toprovide an input electronic signal in transmission line 132, withminimal distortions with respect to the output electronic signal thatwas provided to the pre-compensator. That is, the input electronicsignal in transmission line 132 of the receiver is approximately aduplicate of the output electronic signal in transmission line 110 ofthe transmitter.

[0023] The optical detector may be an avalanche photodiode or morepreferably a PIN photodiode or any other type of low noise photodetector. The post-compensator may include electronic circuits forlinearizing signals containing even order distortions. Alternatively, orin addition, the carrier frequencies for critical signals may be keptwithin a range of one octave so that all second order distortions causedby these carriers are outside of the one octave range of frequencies andthe post-compensator may include a pass band filter for filtering outthese second order distortions. Band pass circuits for performing suchfiltering are well known in the art. Alternatively or in addition, evenorder distortion could be reduced by providing circuitry for providingeven order distortions in the pre-compensator and/or by using duelsignal paths with one inverted signal as in the patent by Glaabdiscussed above in the background.

[0024] In addition to distortions due to laser beam modulation andtransmission through the optical fiber, pre-compensator 111 andpost-compensator 131 may also include circuitry to compensate fordistortions due to optical amplifiers and distortions due to electronicamplifiers, optical detectors, modulators, demodulators, and otherelectronics which may cause odd or even order distortions in thetransmission link.

[0025] Alternatively, the DBF laser shown in FIG. 1 could be replaced bydiscrete laser and a discrete laser beam modulator as shown below inFIG. 2. If properly selected a combination of separate laser andmodulator can produce an optical signal with minimal second orderdistortion due to laser beam modulation. However such a system is moreexpensive and complex.

[0026]FIG. 1 shows in-line compensators rather than the more commonparallel compensators as shown below in FIG. 2. In-line compensators arepreferred because they operate at higher frequencies and such circuitsare simpler.

[0027]FIG. 2 illustrate a portion of a node 200 of another exampleembodiment of the invention, in which an optical transmitter 201transmits optical signals and an optical receiver 202 receives opticalsignals from other nodes, from a common optical fiber 203. A wavelengthdivision multiplexer/demultiplexer (WDMD) connects between the commonoptical fiber 203 carrying a multitude of optical signals in eachdirection, the signals carried in the same direction having differentrespective wavelengths, and multiple bidirectional optical fibers 204and 205 each carrying bi-directional signals of one single respectivewavelength. That is, the forward signal and the return signal in eachbidirectional fiber such as fiber 204, have approximately the samewavelength. The wavelengths of the signals in fiber 204 are differentthan those in fiber 205. Optical splitters 206 and 207 each direct arespective output optical signal from optical transmitter 201 through aoutput optical fiber 208 and 209 respectively into a respective singlewavelength fiber 204 and 205. The optical splitters also each direct arespective input optical signal from respective single wavelength fibers204 and 205 through input optical fibers 210 and 211 respectively tooptical receiver 202. The optical output path from the lasers mayinclude optical isolators (not shown) so that the input signals do notaffect the modulation or wavelength of the laser beams.

[0028] A transmission line 220 provides an electronic output informationsignal to optical transmitter 201. In the optical transmitter the signalis split by splitter 221 into two transmission lines 222 and 223 whichdirects the output signal into two transmission circuits which may beidentical except for an inverter 224.

[0029] Some details of the transmitter circuit are only shown in one ofthe identical circuits to simplify the figure, and Identical portions ofthe transmission circuits will be described with reference to only oneof the circuits to simplify the description. Each transmission circuitincludes a modulator 225 to modulate the electronic output signal. Themodulated signal is distorted by a pre-compensator 227 to compensate thesignal for distortions that occur later in the signal path, includingthose due to laser modulation and transmission through the opticalfibers. The distorted output signals are amplified by preamplifier 229and one or more power amplifiers 231.

[0030] Laser transmitter 233 converts the distorted, modulated,amplified output signal into an optical output signal. The amplifiedoutput signal is used to modulate a laser beam 233 produced by laser235, and optics 237 direct the laser beam into a proximate end 239 ofoptical fiber 208. Each pre-compensator 227, 228 includes an odd orderpre-compensator 250 to distort of the electronic output signal tocompensate for odd order distortions in the output signal due to boththe modulation of the laser beam and the transmission of the opticaloutput signal through the optical fiber tree network. Also eachpre-compensator includes an even order pre-compensator 252 fordistorting the electronic output signal to compensate for even orderdistortions due to both the modulation of the laser beam and thetransmission of the optical output signal through the optical fiber treenetwork. The pre-compensator 227 also includes a delay circuit 254 thatdelays a non-distorted output signal with the same delay that isproduced by the odd order pre-compensator and even order compensator.The compensators 250 and 252 provide paths parallel to the main paththrough delay 254. Thus, these are not in-line compensators, as ispreferred in the invention, but which may be used where the advantagesof in-line compensation is not critical.

[0031] Optical transmitter 201 also includes a controller 260 to providecontrol signals in the output signal depending on the input signal. Theinput signal also includes such control signals generated at anothernode (not shown) depending on the output signal of this node, asreceived by the other node (not shown).

[0032] Laser control 262 uses the control signals in the input signal tocontrol the operation of laser 235. Compensation adjuster 264 also usesthe control signals in the input signal to adjust the compensationprovided by the pre-compensator in order to minimize the distortion inthe other input signal of the other local node. Manual input 265 at thefront panel of the transmitter provides adjustment for initiallycalibrating the compensation adjustment.

[0033] Optical receiver 202 similarly, includes two reception circuitsthat are connected at the end of the reception circuits by a combiner248 to form the electronic input signal. The two reception circuits maybe identical except for inverter 270 and equalizer 271 each at the endof one of the reception circuits. Again, some details are only shown inone of the reception circuits and the identical portions of thereception circuits will be described with reference to only one of thecircuits in order to simplify the description. Each reception circuitincludes a photo detector 272 connected to an input fiber 210. The photodetector converts an optical signal in the optical fiber into anelectronic input signal. The electronic signal is amplified bypre-amplifier 274 and one or more power amplifiers 276. The amplifiedsignal may include distortions due to different lengths of opticalfiber, because the same optical signal may be transmitted to more thanone local node, and the length of the optical fiber between thetransmitting and receiving nodes typically varies for differentreceiving nodes. Post compensator 278 linearizes the input signal tocompensate for the difference between the actual transmission length andthe transmission length for which the signal is pre-compensated for. Thelinearized signal is filtered by a band pass filter 280 to removeintermodulation signals outside of the frequency band of the signal.Even though the system preferably uses an inverted signal dual link tominimize even order distortions, the even order distortions due todirect laser modulation, photo detection, odd order distortioncompensation, and electronic amplification will not be exactly equal andbecause the wavelengths of the dual signals are different even orderdistortions due to transmission through the fiber and due to opticalamplification will be different so that further reductions of even orderdistortions may be significant. Preferably, the output and input signalsare multi-carrier multilevel QAM signals in which all the carrierfrequencies of critical signals (e.g. analog television signals) arewithin an octave, so that most even order distortion can be filtered outof the signal. More preferably, all the carrier frequencies are withinhalf an octave so that fourth order distortions can also be filteredout. The signal may also include less-critical signals (e.g. digitalsignals) with carrier frequencies that are outside of the octave offrequencies used for the critical signals and these less-criticalsignals would not be transmitted through the band pass filter. Theportions of the reception circuits for handling the less-criticalsignals would be essentially the same as for handling the criticalsignals except pass band filtering is not as critical. The filteredsignals are demodulated by demodulator 282 and then the demodulatedsignal is equalized by equalizer 271 and combined by combiner 248 withanother input signal so that even order distortions are further reduced.

[0034] Post compensators 278 and 279 may be identical, but only detailsof post-compensator 278 are shown to simplify the illustration. Eachpost-compensator includes post-compensator 290 for compensating for oddorder distortions and post-compensator 291 to compensate for even orderdistortions, and delay 292 to delay the non-linearized signal the sameamount as the delay of the post-compensators. The post-compensators areonly required in systems where signals are transmitted to more than onelocal node that may have different respective transmission distances.Also, it may be convenient to compensate for distortions due tocomponents in the receiver using a compensator at the receiver so thatthe transmitting node does not have to provide different compensationfor different receiving nodes. Again the post-compensators 290, 291 inFIG. 2 form a parallel circuit with delay 292 so that they are not thein-line post-compensator 131 shown in FIG. 1 that is preferred, but ifthe advantages of inline post-compensation are not required or critical,then the parallel arrangement shown can be used.

[0035] Previously, odd order distortions of optical signals due totransmission through optical fiber have not been recognized or have beenignored because distortions due to laser modulation and even orderdistortions due to transmission through the optical fiber overwhelm theodd order distortions due to transmission through the fiber. Theapplicants have discovered that reductions in distortion due to lasermodulation along with the large reduction in even order distortions inthe circuits of this embodiment result in a signal where the odd orderdistortions due to optical transmission through zero dispersion fiberare significant and in the invention such odd order distortion is alsocompensated for.

[0036] The dual link inverted signal scheme herein, greatly reduces evenorder distortions. By including circuits for amplification and odd ordercompensation within the dual circuits, even order distortions due tothese circuits can also be greatly reduced. Similarly providing all thecarrier frequencies within one octave or even better providing all thecarrier frequencies within one half of an octave also greatly reduceseven order distortion. Also, pre-compensation and/or post-compensationfor even order distortion greatly reduces even order distortions. Theinventors recognize that each of these methods has advantages anddisadvantages so that by combining two or more of these methods reduceddistortion can be achieved in real systems. The dual link scheme isindependent of changes to the length of the common fiber, and eliminateshigher order distortions, but does not eliminate all even orderdistortion because the electrical circuits in the respective links aredifferent. Electronic parts such as lasers, photo-detectors and otherelectronic circuits vary within specified tolerances. The filteringscheme is also independent of changes in the length of the common fiber,but is only practical for eliminating second and possibly fourth orderdistortions. Much of the higher order even-order distortions remainafter filtering. Pre-compensation for even order distortions can beadjusted for the length of the fiber to minimize distortions, butcircuits which precisely cancel out higher order even order distortionsare complex. Also in a CATV system some of the nodes are at differentdistances from the head end so that some post-compensation foreven-order distortion is needed and that also needs to be adjustable forchanging lengths of the common fiber when the system is rerouted. Thelengths of the cables are often changed due to construction andupgrading. By combining these methods together even order distortionscan be minimized in the system.

[0037] FIGS. 3-5 illustrate an example embodiment of the broadbandcommunication system of the invention. In FIG. 3, a head-end 300includes a multimedia access controller 301 communicating with atelephone gateway 302, a computer gateway 303, and a television gateway304. The telephone gateway provides telephone communications with thetelephone network so that customers connected to the broadband systemcan communicate by telephone with persons or computer systems which areconnected to the telephone network outside of the broadband system. Thecomputer gateway provides high speed communications with computersystems such as the internet. The telephone gateway can also be used forlower speed access to such computer systems. The television gatewayreceives television programs, for example, by satellite download fromtelevision studios. In addition, the gateway may provide televisionprograms from one of the broadband system customers for unlink fordistribution outside of the system. The television gateway may alsoprovide interactive television for customers of the broadband network.

[0038] Electronic information signals are routed from the accesscontroller to modulators 308 which modulate carrier signals of differentfrequencies with respective information signals. The modulated signalsare combined by combiners 309 to provide a multi-carrier signal. Themodulators and combiners may be combined into one or more circuits orseparated as shown. The multi-carrier modulated signal is then routed toin-line pre-compensators 310 which distort the modulated electronicinformation signals to compensate for odd order distortions due to lasermodulation and optical fiber transmission. The pre-compensated signalsare routed to laser transmitters 311-314 where the pre-compensatedsignals modulate respective laser beams of respective lasers to producerespective optical information signals. The optical information signalsare routed to wavelength division multiplexers/demultiplexers WDMD's318-319 which combine the laser beams into common optical fibers 328,329 and transmit them to respective hubs described below. The WDMD'salso separate optical signals received from the hubs. The receivedsignals are routed to optical receivers 320-323 which convert thereceived optical signals into respective electronic information signals.The received electronic signals are routed to tuners or separators 324to separate the signals and then to demodulators 325 which convert thesignals to base band signals. Alternatively, the separators anddemodulators may be combined into one or more circuits. The base bandsignals are routed to access controller 301 where they are used tocontrol the access controller or are provided to the correct gateway.

[0039]FIG. 4 shows a hub 340 connected to local nodes 341-343. The hubincludes a common WDMD 345 for communication through a common fiber tothe head-end. The common WDMD communicates with respective WDMD's346-347 for each local node connected to the hub.

[0040] Local nodes 341-343 may be identical, but relevant details areonly shown for local node 341 for simplicity or drawing and description.Local node 341 includes a wavelength division demultiplexer (WDD) 350for the local node. WDD 350 separates optical signals according to lightwavelength. The separated optical signals are routed to opticalreceivers 351-352 which convert the optical signals to forwardelectronic signals. The forward electronic signals are routed topost-compensators 353 which linearize the forward electronic signals tocompensate for differences in the length of transmission through opticalfibers for different local nodes. The forward electronic signals arethen transmitted through coaxial cable tree networks (369-370) tocustomer interface units of FIG. 5, described below.

[0041] In the local node, return signals from the customer interfaceunits are separated from the forward signals in the coaxial cablenetworks by respective diplex filters 357-358. The diplex filters may bepass band filters where the return signals have frequencies within adifferent band than the forward signals. The return signals aremodulated multi-carrier signals from the customer interface unitsdescribed below. The separated electronic return signals are routed topre-compensators 360 for distorting the return signals so as tocompensate for odd order distortions due to subsequently modulating alaser beam with the return signals and transmitting the modulatedoptical signal in the laser beam through fiber optical fiber. Lasertransmitters 361-362 produce respective laser beams which are modulatedby respective return signals. The laser beams have different opticalwavelengths and are routed to wavelength division multiplexer WDM 363for combination into a single common fiber 348 for transmission backthrough hub 340, and then back to head-end 300.

[0042]FIG. 5 illustrates a coaxial tree network 380 for routing signalsbetween the local node shown in FIG. 4 and customer interface units381-385. Each customer interface unit connects between the coaxial cablenetwork and a respective customer's television network 390, computernetwork 392, telephone network 391, and appliance network 393.

[0043] The invention has been disclosed with reference to specificpreferred embodiments, to enable those skilled in the art to make anduse the invention, and to describe the best mode contemplated forcarrying out the invention. Those skilled in the art may modify or addto these embodiments or provide other embodiments without departing fromthe spirit of the invention. Thus, the scope of the invention is onlylimited by the following claims:

We claim:
 1. A pre-compensation circuit for laser communications throughan optical fiber, comprising: signal input means for providing anelectronic signal for information communications; in-linepre-compensation means to electronically distort the electronic signalprior to using the electronic signal to modulate a laser beam to convertthe electronic signal to an optical signal, for compensating for oddorder distortion due to transmitting the optical signal through opticalfiber due to the interaction between the laser beam and the dispersionof the optical fiber; and signal output means for providing thedistorted electronic signal to means for modulating the laser beam. 2.The circuit of claim 1 in which: the pre-compensation means are also forcompensating for distortions due to modulation of a directly modulatedlaser; the pre-compensation means are also for compensating fordistortions due to modulation of a distributed feedback laser; thepre-compensation means are also for compensating for odd orderdistortion of the optical signal due to modulating the laser beam; thepre-compensation means are also for compensating for distortions due totransmission through standard single mode fiber with a zero dispersionat approximately 1310 nm; the pre-compensation means are also forcompensating for distortions due to transmission of a laser beam with awavelength approximately between 1500 and 1610 nm through fiber; thepre-compensation means are also for dynamically adjusting the distortionto compensate for different lengths of transmission through opticalfiber; the adjusting can be made from a front panel; the adjustingincludes providing an adjustment signal to the pre-compensator; thepre-compensation means are also for compensating for even orderdistortions due to using the electronic signal to modulate the laserbeam; the pre-compensation means are also for compensating for evenorder distortions due to transmitting the electronic signal through theoptical fiber.
 3. A pre-compensation circuit for laser communicationsthrough an optical fiber, comprising: signal input means for providingan electronic signal for information communications; precompensationmeans to electronically distort the electronic signal prior to using theelectronic signal to modulate a laser beam to convert the electronicsignal to an optical signal, for compensating for odd order distortionof the optical signal due to transmitting the optical signal throughoptical fiber; means for dynamically adjusting the distortion fordifferent lengths of optical fiber to reduce the distortion of theoptical signal at a receiver at a distal end of the fiber from thelaser; signal output means for providing the distorted electronic signalto a laser for producing the laser beam.
 4. The pre-compensation circuitof claim 3, in which: the adjusting means are also for adjusting thedistortion during operation of the pre-compensation circuit; theadjusting means are for adjusting the distortion from a front panel; thepre-compensation means include an in-line pre-compensation circuit; thepre-compensation means are adapted for compensating for odd orderdistortion of the optical signal due to modulating the laser beam; thepre-compensation means are adapted for automatically adjusting for thedistortion to reduce the distortion of the optical signal at an opticalreceiver depending on a return signal from the receiver of the opticalsignal at a distal end of the optical fiber from the laser to minimizedistortion in an electronic information signal output from the opticalreceiver with respect to the input electronic information signal; andthe pre-compensation means are adapted for providing a signal indicatingthe length of the optical fiber for which the distortion is adjusted tocompensate.
 5. A laser transmitter comprising: signal input means forproviding an electronic signal; a laser for producing a laser beam witha predetermined wavelength of light; in-line pre-compensation means toelectronically distort the electronic signal prior to using theelectronic signal to modulate the laser beam to convert the electronicsignal into an optical signal, for compensating for odd order distortiondue to transmitting the optical signal through zero dispersion fiber;means for modulating the laser beam; an optics system for directing themodulated laser beam into the optical fiber.
 6. The laser transmitter ofclaim 5 in which: the laser is a directly modulated laser so that thelaser and means to modulate the laser beam are integral; the laser is adistributed feedback laser; and the laser has a wavelength approximatelybetween 1500 and 1610 nm
 7. An information communication system,comprising a network of optical fibers; a first node and second localnode communicating through an optical fiber network, each comprising:means for providing an output electronic information signal; in-linepre-compensation circuit means to electronically distort the outputelectronic signal to produce a distorted electronic signal, forcompensating for odd order distortion due to transmitting the opticaloutput signal from the first node to the second node through the opticalfiber network; a laser for producing the output laser beam with apredetermined wavelength of light; means to modulate the output laserbeam with the distorted electronic signal; an optics system fordirecting the modulated output laser beam into a first one of theoptical fibers as an output fiber to transmit the optical output signal;an optical detector for converting an optical input signal into an inputelectronic signal; and additional compensation means for electronicallycompensating for even order distortions due to using the electronicsignal to modulate the laser beam, and for compensating for even orderdistortions due to transmitting the electronic signal through theoptical fiber from the first node to the first second local node; andwhereby optical signals are communicated bidirectionally between thefirst node and the second node and the optical information signal hashigher distortion at the means to modulate than at the optical detector.8. The system of claim 7 in which: the system further comprises: a thirdlocal node; and an optical splitter for providing the output opticalsignal from the first node to both the second local node and the thirdlocal node and providing input optical signals from both the secondlocal node and the first local node to the first node; and in which: thelength of optical fiber between the first local node and the third localnode is different than between the first local node and the second localnode; and the system further comprises post-compensation means includingan electronic circuit for compensating for odd order distortion due tothe difference in length between the distance from the first local nodeto the second local node and the distance between the first local nodeand third local node.
 9. A cable television network, comprising: anetwork of optical fibers; and a head-end including: program means forreceiving a multitude of television programs; gateway means for datacommunications with other types of networks including a telephonenetwork for receiving and transmitting telephone communications andcomputer networks for transmitting and receiving computer information;modulator means for modulating the television programs and the datacommunications received from the gateway means with multiple carrierseach television program having a carrier at a different respectivefrequency and the data communications using multiple carriers withdifferent respective frequencies; combiner means for combining multiplecarriers into one or more electronic forward signals; a respective laserfor each forward signal, for producing a respective forward laser beamwith a predetermined wavelength of light; respective means formodulating each laser beam with the electronic forward signal to convertthe electronic forward signal to a respective optical forward signal;respective in-line pre-compensation means for each electronic forwardsignal, including an electronic circuit to distort the electronicforward signal prior to using the electronic signal to modulate theforward laser beam, for compensating for odd order distortion due totransmitting the optical forward signal through optical fiber; arespective optics system for each laser, for directing the modulatedforward laser beam into respective ones of the optical fibers for eachlaser beam; respective detector means for converting one or more opticalreturn signals from respective ones of the optical fibers, intoelectronic return signals; means for providing the electronic returnsignals as data communications to the gateway means for transmitting thereturn signals as telephone communications and computer information; andthe network further comprising: a multitude of separate coaxial cablenetwork trees; a plurality of local nodes, each connected to one or moreoptical fibers and one or more coaxial cable networks, each local nodeincluding: respective means for converting one or more optical forwardsignals from respective ones of the optical fibers, into respectiveelectronic forward signals in one or more coaxial cable networks; returnsignal separation means including a respective diplex filter for eachcoaxial cable network, for separating return data signals from theelectronic forward signals in the respective coaxial cable network; areturn laser for each electronic return signal, for producing arespective return laser beam with a predetermined wavelength of light;return pre-compensation means for each electronic forward signal,including an electronic circuit to distort the electronic return signalprior to using the electronic signal to modulate the return laser beamto convert the electronic return signal into an optical return signal,for compensating for odd order distortion due to modulating the returnlaser beam and for compensating for odd order distortion due totransmitting the optical return signal through a optical fiber; amultitude of customer interface units connected to the coaxial cablenetwork trees for receiving the forward information signals and fortransmitting return data signals, including; means for connecting atelevision display to the coaxial cable network tree for displaying thetelevision programs; means for connecting telephone equipment to thecoaxial cable network for receiving and transmitting telephonecommunications and for connecting computer equipment to the coaxialcable network for transmitting and receiving computer information.
 10. Amethod for optical communication comprising: receiving an electronicinformation signal; adjusting a pre-compensation circuit for the lengthof an optical fiber through which the information signal will betransmitted; precompensating the electronic information signal tocompensate for odd order distortion of the optical signal due totransmitting the optical signal through optical fiber, so that the oddorder distortion at a receiver at a distal end of the optical fiber fromthe laser is reduced; producing a laser beam using a laser; modulatingthe laser beam using the electronic information signal to produce anoptical information signal; transmitting the optical information signalthrough the optical fiber to an optical receiver; whereby the opticalinformation signal has lower odd order distortion at the opticalreceiver than when it was modulated.
 11. The method of claim 10 inwhich: the method further comprises adjusting the distortion dependingon the length of the optical fiber; the method further comprisescompensating for odd order distortion due to modulating the laser beamwith the electronic information signal; and the method further comprisescompensating for even order distortion of the optical signal due tomodulating the laser beam and for even order distortion of the opticalsignal due to transmitting the optical signal through the optical fiber.